I installed Windows 8.1 on my laptop and it worked well for a couple of weeks, but now suddenly the brightness locked. When I try to adjust it with the F11 and F12 keys it stays the same.
In such situations the following steps could address this issue:
Go to Device Manager by Control Panel>>Device Manager
Open the Display Adapters (see image below)
Look for your display adapter driver: it's either Intel or AMD. In this illustration on a Yoga 13, it is Intel.
Then, restart your computer and install the following display/ video card driver instead. http://mobilesupport.lenovo.com/us/en/products/laptops-and-netbooks/lenovo-g-series-laptops/lenovo-g500-notebook